Phase modulation schemes are very effective and are therefore widely used in communication systems. A simple example of a phase modulation scheme is quaternary phase shift keying (QPSK). FIG. 1 shows a constellation diagram 100 that illustrates how QPSK maps two-bit digital data to one of four phase offsets. FIG. 2 shows a typical QPSK (or in-phase/quadrature (I/Q)) modulator 200 used to generate a phase-modulated signal. This technique relies on orthogonal signal vectors to realize the phase offsets—an inherently linear technique, since it depends solely on the matching of these orthogonal signals.
The I/Q modulator provides a straightforward approach to generating phase-modulated signals that is also suitable for more complex schemes such as Wideband Code Division Multiple Access (WCDMA) and Orthogonal Frequency Division Multiplexing (OFDM) systems. It is also possible to generate the phase-modulated signals using a phase-locked loop (PLL). This approach offers reduced circuitry and lower power consumption and, as a result, finds widespread use in narrowband systems.
In some systems direct modulation may be applied to a voltage controlled oscillator (VCO). This is a difficult task since the VCO gain depends on multiple variables. It would therefore be advantageous to provide systems and methods for accurately setting and tracking the VCO gain, KFM.